Simoncelli, 24, crashed on the second lap and was hit by the bikes of Colin Edwards and Valentino Rossi as he slid across the track on his Honda.

His helmet was knocked off in the incident and the season's penultimate race was stopped immediately with a red flag and then cancelled.

Simoncelli lay stricken on the track before being taken to the circuit's medical centre where he was pronounced dead.

MotoGP's medical director Michele Macchiagodena was close to tears as he reported the details of the death of "a friend" to a news conference at the circuit.

"He suffered a very serious trauma to the head, to the neck and the chest. When our medical staff got to him he was unconscious. In the ambulance because there was a cardiac arrest they started CPR, cardiac pulmonary resuscitation," said Macchiagodena.

"In the Medical Centre he was intubated and it was possible to take off some blood from the thorax. The CPR was continued for 45 minutes because we tried to help him for as long as we thought it was possible. Unfortunately it was not possible to help him and at 16:56 (local time) we had to declare he was dead."

Simoncelli's death is the first in the premier class since another Honda rider, Daijiro Kato, was killed at the 2003 Japanese Grand Prix.

Shoya Tomizawa also died in a similar crash to Simoncelli in Moto2, the class below MotoGP, last year.

Simoncelli, known for his shaggy afro haircut and fun-loving personality, began in the 125cc class in 2002 before moving up to the 250cc class in 2006. He won the 250cc world title in 2008.

MotoGP bosses and riders have been working hard to improve safety following Tomizawa's death at the San Marino Grand Prix in September 2010 but have acknowledged there is little more they can do given the implicit danger in motorcycling, especially when riders are hit by other bikes.

All Italian sports events on Sunday will observe a minute's silence in memory of Simoncelli, the Italian Olympic Committee said in a statement.

Simoncelli was a big fan of soccer club AC Milan, who released a statement offering their condolences.